How it's calculated
Paint is sold by volume but used by area, and every can lists how far a gallon goes. The calculator finds the wall area of a rectangular room, takes out the doors and windows you won't paint, optionally adds the ceiling, multiplies by the number of coats and divides by the coverage per gallon (or per liter). The result is rounded up to whole containers.
Coverage depends on the product. Sherwin-Williams lists 350–400 sq ft per gallon for SuperPaint Interior Latex Flat Sherwin-Williams, and Benjamin Moore lists 400–450 sq ft for Regal Select Interior Matte Benjamin Moore. The calculator starts at 350, the low end of those ranges, so it errs toward having enough; type in the figure from your own can. Both manufacturers recommend two coats, which is the default.
The formula
- Wall area = 2 × (room length + room width) × wall height.
- Openings = doors × door area + windows × window area (never more than the wall area).
- Area to paint = wall area − openings, plus length × width if you include the ceiling.
- Paint needed = area to paint × coats ÷ coverage per gallon (or per liter).
- To buy = paint needed, rounded up to a whole gallon (or liter). Cost = containers × price.
- Metric coverage: 350 sq ft per US gallon is about 8.6 m² per liter, using NIST's exact factors (1 ft = 0.3048 m; 1 US gallon = 231 in³ = 3.785411784 L) NIST.
Worked example
Take a 12 × 14 ft bedroom with 8 ft walls, one 3 × 7 ft door and two 3 × 5 ft windows, painted with two coats at 350 sq ft per gallon. The area to paint is 365 sq ft, so the room needs 2.09 gal of paint: buy 3 gallons, which costs $120 at an example $40 a gallon.
Paint to buy: 3 gallons (2.09 needed)
Painting the ceiling with the same paint raises that to 4 gallons. With a paint that covers 450 sq ft per gallon, the walls alone need 1.62 gal.
Assumptions and limitations
- Coverage is a best case. Data-sheet coverage is for smooth, sound surfaces. Textured, porous or unprimed walls, new drywall and strong color changes use more paint, and spraying wastes some.
- One paint for everything. Trim, doors and ceilings often use a different product or sheen; estimate those separately.
- Rectangular rooms. For sloped ceilings or gable walls, add the extra triangle areas yourself (the square footage calculator handles triangles).
- Whole containers. The count rounds up to whole gallons or liters. Quarts (a quarter gallon) can top up a small shortfall.
- Primer not included. If the data sheet calls for a primer on your surface, estimate it the same way with the primer's coverage.
- Prices are examples. Enter your own.
Hanging new drywall first? The drywall calculator counts sheets, screws and joint compound for the same room.